Figure 1. Path diagram representation of the hypothetical regression model. The top panel of the Figure represents the standard multiple regression model with two predictors. The bottom panel of the figure represents the same regression model but includes four auxiliary variables (qol2, qol3, age, ed). Note that the auxiliary variables are correlated with (a) each other, (b) the substantive predictors (qol1 and tx), and (c) the residual term.